Method for producing a hot rolled strip and hot rolled strip produced from triplex lightweight steel

ABSTRACT

The invention relates to a method for producing a hot strip from a triplex lightweight steel, wherein a melt is cast into a roughed strip and the latter is subsequently rolled into a hot strip. For this purpose, it is provided that the melt is cast in a horizontal strip casting facility under conditions of a calm flow and free of bending into a roughed strip in the range between 6 and 20 mm and is subsequently rolled into hot strip having a degree of deformation of at least 50%.

The invention relates to a method for producing a hot strip from a triplex lightweight steel, wherein a melt is cast into a roughed product and the latter is then rolled into a hot strip.

Triplex lightweight steels cannot be produced by using the common continuous casting route, i.e. continuous casting of the melt into a slab or thin slab which is rolled either in-line or separately into a hot strip, with the required properties.

The reasons for that reside in the fact that the slab or thin slab, produced by continuous casting, has macro segregations and forms shrink marks. Moreover, the roughed product has a very coarse grain and casting with casting powder poses problems because of the high aluminum content of the ferritic steel.

Triplex lightweight are known, for example, from DE 10 2005 057 599 A1 and DE 102 31 125 A1. They are characterized by a 3-phase microstructure α(γ) κ and in view of the high proportion on alloying components with a specific weight below the specific weight of iron by a respectively low weight.

Moreover, this steel is able to achieve a beneficial combination of high strength paired with high elongation in the structure, when treated accordingly.

Triplex lightweight steels are therefore especially suitable for the automobile industry which demands such combinations for certain body parts in order to be able to show proof of deformation reserves in the event of an accident.

DE 100 60 948 C2 discloses a production of hot strips from steel having a high manganese content with 12 to 30 weight-% of manganese and up to 3.5 weight-% of each of aluminum and silicon in such a way that the steel melt is cast in a double-roller casting machine to form a roughed strip close to the final dimensions with a thickness of up to 6 mm, and subsequently the roughed strip is hot rolled continuously preferably in a single pass.

The stated upper limit for the thickness with 6 mm cannot be achieved with existing facilities. The maximum thickness that can actually be adjusted is typically 4 mm, in exceptional cases maximal 5 mm.

This known method has the advantage that macro segregations are reduced, shrink marks are suppressed, and the problem associated with casting powder is not relevant.

It is, however, disadvantageous that the small starting thickness of the roughed strip permits only a small hot deformation degree during rolling, when a thickness of 2-3 mm of the hot strip is desired. This thickness range, for example, is however of interest for the use of the hot strip as lightweight component in the area of the chassis, e.g., as transverse control arm or longitudinal control arm, on the one hand. On the other hand, a cold strip with a thickness of, for example, 1.0-1.8 mm can be produced from a hot strip of a thickness of 2-3 mm at a degree of deformation of 40-50% and can be used, e.g., for B pillars or side rails at the front or back. A small hot deformation degree means, however, coarse grain which adversely affects ductility and thus the formability.

It is therefore an object of the invention to provide a method for producing a hot strip from a triplex lightweight steel which method is able to realize a fine grain in the hot strip of 2-3 mm thickness while maintaining the benefits of the double-roller casting machine.

This object is attained by a method in which the melt is cast in a horizontal strip casting facility under conditions of a cairn flow and free of bending into a roughed strip in the range between 6 and 20 mm, and subsequently rolled into a hot strip with a degree of deformation of at least 50%.

The proposed method has the advantage that the benefits of the known double-roller casting machine, like reduction of macro segregations, suppression of shrink marks, and prevention of the problem associated with casting powder, are retained, even when the ferritic steel has high Al contents, when using a horizontal strip casting facility, and furthermore the thickness of the roughed strip is significantly above the thickness of a roughed strip produced by means of a double-roller casting machine.

This affords the possibility to attain sufficiently high degrees of deformation in terms of adjusting a fine grain in the microstructure of the hot strip; this is true in particular when the hot strip has a thickness in the range of 2-3 mm. Triplex lightweight steels do no show a complete γ-α transformation so that there is a tendency to form coarse grain which can be reversed through a sufficient degree of deformation during hot rolling.

A further advantage involves a very rapid cooldown and solidification of the deposited melt in a strip casting facility. The triplex lightweight steel exhibits its positive properties also from the nano-sized dispersed “kappa” carbides in the austenite matrix. The rapid cooldown of the melt promotes the fine distribution and a slight growth of the carbides. Therefore, the advantages of the proposed casting process are maintained even after hot rolling and annealing as a result of the beneficial carbide distribution.

In terms of the process, it is proposed to achieve the calmness of flow by using a co-moving electromagnetic brake which ensures that in the ideal case the speed of the melt feed equals the speed of the revolving conveyor belt.

The bending considered disadvantageous during solidification is prevented by supporting the underside of the casting belt receiving the melt upon a multiplicity of rollers placed side-by-side. The support is reinforced by generating in the region of the casting belt a negative pressure to press the casting belt firmly against the rollers.

In order to maintain these conditions during the critical phase of solidification, the length of the conveyor belt is selected in such a way that the roughed strip is substantially solidified at the end of the conveyor belt before the latter is deflected.

The end of the conveyor belt is followed by a homogenization zone which is utilized for a temperature equalization and possible stress relief.

Rolling of roughed strip into hot strip may be realized either in-line or separately off-line. Before off-line rolling, the roughed strip after production and before cooldown can either be coiled directly in hot state or cut into panels. The strip or panel material is then reheated after possible cooldown and unwound for off-line rolling or reheated as panel and rolled.

Beneficial technical values are attained when the degree of deformation is>70% and a mean grain size of>6 ASTM can be adjusted.

A preferred grade for the triplex lightweight steel includes high Mn contents of>18 weight-%, with high Al contents of>8 weight-%, with C contents of>0.6 weight-%, and small Si contents with<0.25 weight-%.

Optionally one or more precipitation-forming elements of type B, Ta, Zr, Nb, V, Ti, Mo and W may be added collectively at a maximum of 2 weight-%. 

1.-15. (canceled)
 16. A method for producing a hot strip from a triplex lightweight steel, comprising the steps of: casting a melt in a horizontal strip casting facility under conditions of a calm flow and free of bending to form a roughed strip having a thickness in a range between 6 and 20 mm; and rolling the roughed strip into a hot strip with a degree of deformation of at least 50%.
 17. The method of claim 16, further comprising feeding the melt into the horizontal strip casting facility at a speed which equals a speed of a revolving conveyor belt of the horizontal strip casting facility.
 18. The method of claim 17, further comprising subjecting all surface elements of a strand shell, forming at the start of solidification, of a strip extending across a width of the conveyor belt to approximately same cooldown conditions.
 19. The method of claim 17, wherein the melt on the conveyor belt has substantially solidified at an end of the conveyor belt.
 20. The method of claim 16, further comprising passing the roughed strip through a homogenizing zone after complete solidification and before starting a further treatment.
 21. The method of claim 20, wherein the further treatment involves cutting the roughed strip into panels.
 22. The method of claim 21, further comprising heating the panels to a rolling temperature, and subsequently subjecting the panels to a rolling process.
 23. The method of claim 20, wherein the further treatment involves a coiling of the roughed strip.
 24. The method of claim 23, further comprising unwinding the roughed strip, heating the roughed strip to a rolling temperature, and subsequently subjecting the panels to a rolling process.
 25. The method of claim 24, further comprising reheating the roughed strip before being the unwinding step.
 26. The method of claim 16, further comprising subjecting the roughed strip in line to the rolling step, and further comprising coiling up the roughed strip.
 27. The method of claim 16, wherein the degree of deformation is >70% during hot rolling.
 28. The method of claim 17, further comprising applying a negative pressure in an area of the conveyor belt.
 29. The method of claim 17, further comprising supporting an underside of the conveyor belt by a plurality of rollers in side-by-side relationship.
 30. Hot strip from a triplex lightweight steel, having a mean grain size of>6 ASTM.
 31. The hot strip of claim 30, wherein the triplex lightweight steel has a chemical composition in weight-% of <0.6 C; >18 Mn; >8 Al; <0.25 Si, remainder iron including unavoidable steel-incidental elements.
 32. The hot strip of claim 31, wherein the triplex lightweight steel has optionally one or more precipitation-forming elements of type B, Ta, Zr, Nb, V, Ti, Mo and W collectively at a maximum of 2 weight-%. 